A Dexcom sensor is a small, wearable device that continuously tracks your glucose levels through a tiny wire inserted just beneath the skin. Instead of pricking your finger multiple times a day, the sensor automatically measures glucose every few minutes and sends readings to your phone, smartwatch, or a separate receiver. Dexcom is one of the most widely used continuous glucose monitor (CGM) brands, and its sensors are used by people with type 1 diabetes, type 2 diabetes, and even those without diabetes who want to understand how food and exercise affect their blood sugar.
How the Sensor Works
Traditional fingerstick meters measure glucose from a drop of blood. A Dexcom sensor works differently: it measures glucose in interstitial fluid, the thin layer of fluid that surrounds your cells just below the skin. A hair-thin wire sits in this fluid and continuously detects glucose levels. The sensor takes a reading every minute and transmits an updated value to your display device every five minutes via Bluetooth.
Because interstitial fluid glucose lags slightly behind blood glucose, there can be a small delay of a few minutes between what a fingerstick would show and what the sensor displays. In practice, this gap is small enough that Dexcom sensors are approved for making insulin dosing decisions without a confirmatory fingerstick. The one exception: if your sensor reading doesn’t match how you feel, a fingerstick is still the right call.
What It Looks Like and Where It Goes
The current Dexcom G7 sensor is roughly the size of a coin, measuring about 1.1 by 0.9 inches and only 0.2 inches thick. That’s 60% smaller than the previous G6 model. It sticks to your skin with an adhesive patch, and most people forget it’s there after the first day.
For adults and children age 7 and older, the sensor is worn on the back of the upper arm. Children between ages 2 and 6 can also wear it on the upper buttocks. These are the only FDA-cleared placement sites, and wearing the sensor elsewhere can affect accuracy.
Applying the sensor takes about a minute. You press an applicator against your skin, click a button, and the thin wire inserts automatically. The G7 has a 30-minute warm-up period before it starts displaying readings, a significant improvement over the G6, which required two hours. Each G7 sensor lasts 10 days, with a 12-hour grace period to swap in a new one.
Alerts and Predictive Warnings
One of the most valued features of a Dexcom sensor is its alert system. You can set custom notifications for when your glucose goes above or below thresholds you choose. There’s also a built-in “Urgent Low Soon” alert that uses predictive modeling to warn you when your glucose is expected to drop to a dangerously low level within the next 20 minutes, giving you time to eat something before you actually go low.
These alerts work around the clock, including while you sleep. For people on insulin, overnight low blood sugar is a real concern, and having a device that wakes you up before it happens can be genuinely life-changing.
Phones, Watches, and Insulin Pumps
Dexcom G7 readings display on compatible iPhones, Android phones, and Apple Watches. A feature called Direct to Watch lets you see your glucose on an Apple Watch (Series 6 or newer, running watchOS 10 or later) even without your phone nearby. This works with both the standard G7 and the G7 15 Day sensor.
The sensor also integrates directly with several insulin pumps to create automated insulin delivery systems. These pumps read your Dexcom data and adjust insulin doses in real time. Currently compatible systems include the Omnipod 5 (a tubeless pump), the Tandem Mobi, the Tandem t:slim X2, and the iLet Bionic Pancreas. In these setups, the sensor effectively closes the loop between glucose monitoring and insulin delivery, reducing how many decisions you need to make throughout the day.
Dexcom G7 vs. G6
The G7 is Dexcom’s current-generation sensor, replacing the G6. The key improvements are practical ones. The G7 sensor is physically smaller (1.1 x 0.9 inches vs. 1.8 x 1.2 inches), warms up in 30 minutes instead of two hours, and combines the sensor and transmitter into a single disposable unit, so there’s no separate transmitter to track or recharge. Both sensors last 10 days per wear.
Stelo: The Over-the-Counter Option
Dexcom also makes the Stelo, a sensor available without a prescription. It’s designed for adults 18 and older who don’t use insulin: people with prediabetes, type 2 diabetes managed without insulin, or anyone curious about how their diet, stress, and exercise patterns affect glucose levels.
The Stelo lasts 15 days per sensor (longer than the G7’s 10 days) but has some important limitations. It doesn’t provide real-time high or low glucose alerts, which means it’s not appropriate for anyone at risk of dangerous blood sugar swings. Its readable range is narrower, covering only 70 to 250 mg/dL compared to the G7’s 40 to 400 mg/dL. And it can’t connect to insulin pumps or smart pens. Think of it as a wellness and awareness tool rather than a medical management device.
Accuracy
CGM accuracy is measured by something called MARD, which stands for mean absolute relative difference. It’s the average gap between what the sensor reads and what a lab-quality blood test shows. Lower numbers mean better accuracy. The Dexcom G7 has a MARD of around 8-9% in outpatient use, which is considered very accurate for a wearable sensor. Even in more challenging clinical settings like hospital intensive care units, the G7 has demonstrated a MARD of 12.5%, outperforming the G6’s 15.2% in the same conditions.
No CGM is perfect, and readings can drift during rapid glucose changes or when you’re dehydrated. But for day-to-day diabetes management, the accuracy is high enough that Dexcom sensors are approved for making treatment decisions, including insulin dosing, without a backup fingerstick.
What a Dexcom Sensor Costs
With insurance, most people pay a copay for a monthly supply of sensors. Without insurance, the retail cost can be significant, typically running several hundred dollars per month for the G7. The Stelo, since it doesn’t require a prescription, is purchased out of pocket and is generally less expensive per sensor, though it still represents an ongoing cost since each sensor is disposable. Many insurance plans, including Medicare, cover the G7 for people with diabetes who meet certain criteria, so checking your specific coverage is worth doing early.